The present invention relates to a piston for an internal combustion engine and a surface treatment method for the piston.
Various compositions of binder resins and solid lubricants have been proposed as lubrication coating compositions for improving the wear resistance and seizing resistance of sliding members such as pistons of internal combustion engines.
Japanese Laid-Open Patent Publication No. 07-097517 discloses one such a lubrication coating composition, which contains 50 to 73 wt % of at least one of a polyamide-imide resin and a polyimide resin as a binder and 3 to 15 wt % of polytetrafluoroethylene, 20 to 30 wt % of molybdenum disulfide and 2 to 8 wt % of graphite as a solid lubricant (i.e. the total content amount of the solid lubricant falls within the range of 27 to 50 wt %) to attain a low friction coefficient and high wear resistance. The seizing resistance of the lubrication coating composition can be improved by the addition of molybdenum disulfide and graphite. However, the film of the lubrication coating composition significantly decreases in strength when more than the necessary amounts of molybdenum disulfide and graphite are added to the coating composition. This leads to an increase in the amount of wear of the coating composition film.
On the other hand, Japanese Laid-Open Patent Publication No. 2008-056750 discloses a double-layer lubrication coating composition for an engine piston, which includes an inner (lower) coating layer formed on a base metal material (e.g. aluminum alloy) of the piston and containing 50 to 79 wt % of at least one of an epoxy resin and a polyamide-imide resin as a binder and 15 to 30 wt % of polytetrafluoroethylene and 5 to 20 wt % of molybdenum disulfide as a solid lubricant and an outer (upper) coating layer formed on a surface of the inner (lower) coating layer opposite to a surface facing the piston base material and containing 50 to 70 wt % of at least one of an epoxy resin and a polyamide-imide resin as a binder, 5 to 20 wt % of boron nitride as a solid lubricant and 15 to 30 wt % of at least one of silicon nitride and alumina as hard particles. This double-layer lubrication coating composition can combine high wear resistance with high seizing resistance.